This invention is in the field of communication satellite systems, and more specifically, in the field of non-geostationary orbit (NGSO) satellite systems.
The commercial success of NGSO satellite communication systems is heavily dependent on the cost and size of user terminals. The cost of the user terminal represents a significant financial hurdle for the establishment of service to the consumer. Small size of the user terminal, especially the outdoor unit comprising the antenna, is very important to ease of installation, appearance and compliance with local building codes.
A well-known problem with high data rate LEO (Low Earth Orbit) satellite communication systems is that the ground terminal requires a high gain antenna that tracks the satellites as they pass overhead. A LEO constellation having several satellites in each orbit plane would require a ground terminal antenna that could scan in two dimensions over all of space over some particular elevation angle. A typical scan pattern for such a system is shown in FIG. 1. From the perspective of a user at a fixed position on earth, the rotation of the earth causes a rising satellite to have a ground track that is slightly westerly from that of the preceding satellite. After a satellite sets, a retrace is required to acquire the next rising satellite. To avoid loss of data, the retrace must be nearly instantaneous. For this reason, the terminal must have two mechanically scanning antennas, with one tracking a satellite while the other antenna is retracing. A single antenna would not be able to retrace rapidly enough to avoid data loss. The need for two antennas translates into high user terminal costs and large size.
Systems using geostationary satellite orbits can operate with lower cost user terminals that use simple fixed pointing antennas. Examples of such antennas (direct broadcast satellite television antennas for example) are common on the rooftops of residential and commercial buildings. The success of direct broadcast satellite systems (such as those marketed under the names DirectTv(trademark) and DirectPC(trademark), for example) is largely dependent on the low cost and small size of the user terminals. GEO systems like these utilize a higher cost space segment in order to reduce the cost of the ground segment (i.e., user terminals). The high cost of the space segment is attributed to launching into GEO orbits and the use of high EIRP transponders. This enables the use of fixed position (non-scanning) antennas of small size in the user terminals.
Despite the user terminal cost advantage of having fixed position antennas, broadband GEO satellite systems have a distinct performance disadvantage relative to LEO (low earth orbit) systems. The latency introduced by the signal propagation delay in GEO systems causes inefficiencies in data communication protocols such as TCP/IP. Therefore, there is recent interest in broadband LEO systems. LEO systems have been proposed with user terminals having antennas, which scan over two axes, as discussed above, resulting in greater size and cost. But, user terminals, which are larger and more costly than those needed for GEO systems, can present an obstacle to consumer acceptance of LEO systems.
This invention achieves lower ground segment cost for LEO systems by providing a system using advantageous LEO orbits in a unique and novel manner. A preferred embodiment of the invention enables small, low cost user terminals by providing a constellation of satellites with substantially continuously repeating ground tracks over the entire earth for user elevation angles greater than zero. This constellation design enables lower cost user terminal antennas by requiring only one axis of scan (elevation) instead of the two axes (azimuth and elevation) usually required for NGSO satellite constellations.
In a preferred embodiment, the invention uses a constellation of satellites that result in reduced cost of the ground segment. The next best thing to a user terminal with a fixed position GEO antenna is one with an antenna that scans in only one axis, rather than the two axes normally required for NGSO antennas. The preferred embodiment of the invention enables use of such an antenna by providing a constellation of satellites in repeating ground track orbits. This type of constellation enables lower cost user terminals while achieving low latency with LEO orbits.
In one embodiment, a satellite communication system includes a constellation of non-geostationary orbiting satellites following a fixed ground track on the earth, and ground terminals having substantially continuous visibility to at least one of the satellites in a single ground track. In this embodiment, each of the satellites follows a single substantially linear track across the sky when observed from the earth. In a preferred embodiment, the system can also include ground terminals, which have an antenna with a single axis of scanning motion. In the system of this embodiment, satellites have repeating ground tracks wherein a satellite completes n orbital revolutions each time the earth rotates through 360 degrees, wherein n is an integer between 2 and 15, inclusive.